1. Field of the Invention
The present invention relates to a structure for mounting a cylinder head cover of an overhead valve-type internal combustion engine having a valve mechanism in a cylinder head.
2. Description of the Related Art
As internal combustion engines in which a cylinder head cover is mounted on a cylinder head by using bolts and coil springs, those which are disclosed in Japanese Utility Model Unexamined Publication Nos. 101449/1981 and 162395/1983 are known.
In the internal combustion engine disclosed in Japanese Utility Model Unexamined Publication No. 101449/1981, each bolt is split into a bolt head and an externally threaded portion, upper and lower end portions of the coil spring are integrally coupled to a lower end of the bolt head and an upper end of the externally threaded portion, a collar portion is formed on an upper end of a rubber boot capable of fitting over an outer periphery of the coil spring, and the rubber boot is fitted and inserted in a through hole formed in a flange portion of the cylinder head cover. The externally threaded portion, the coil spring, and the bolt head are consecutively fitted and inserted in the rubber boot, and after a tip of the externally threaded portion is made to abut against an internally threaded portion in the cylinder head, the bolt head is rotated so as to screw the externally threaded portion into the internally threaded portion, thereby mounting the cylinder head cover onto the cylinder head.
In this device, since the torque applied to the bolt head is transmitted to the externally threaded portion by means of the coil spring, unless the torsional rigidity of the coil spring is very high, the externally threaded portion cannot be securely threadedly engaged in the internally threaded portion and is liable to come off. If the torsional rigidity of the coil spring is increased to overcome this problem, the compressive rigidity of the coil spring also becomes high correspondingly, so that there is a drawback in that the effect of resiliently mounting the head cover declines appreciably.
Meanwhile, with the internal combustion engine disclosed in Japanese Utility Model Unexamined Publication No. 162395/1983, internally threaded portions are formed in a rocker shaft holder for pivotally supporting a rocker arm, coil springs are loosely fitted over external threads which penetrate the cylinder head cover from above to below, and the external threads are threadedly secured in the internally threaded portions, wherein the peripheral edge portion of the cylinder head cover is pressed against the cylinder head through packing by means of the spring forces of the coil springs.
With the with the internal combustion engine disclosed in Japanese Utility Model Unexamined Publication No. 162395/1983, top portions of the external threads substantially project upward from the top surface of cylinder head cover, and the overall height of the internal combustion engine becomes high, so that it is difficult to make the internal combustion engine compact.
In addition, if the amount of projection of the external threads from the cylinder head cover is made small, the rigidity of the coil springs becomes high, and the force with which the cylinder head cover is pressed changes substantially with respect to the change in the amount of screwing in of the external threads, making it difficult to obtain an appropriate pressing force.